Surface reactions monitored on the 'macro'
scale
Reactions followed on the atomic scale.

As an example, the chemistry of hydrogen
on silicon surfaces has been studied in some detail, using a variety
of techniques.

It is possible to image the motion
(diffusion) of hydrogen on silicon at the atomic scale
using a scanning tunneling microscope (STM), which can take an
image in about 18 seconds, operated at an elevated temperature.

Shown here is a brief movie made from individual
frames taken with an STM. It shows a silicon surface mostly covered
with hydrogen atoms and heated to 300 C. The bright moving features
actually are places where the hydrogen is absent. Pairs
of vacancies are stable, but unpaired vacancies can diffuse. Note
the pair of vacancies (at center top) that start paired, then
unpair and later pair again. Other lone vacancies simply wander
during the course of the movie.